Method and apparatus for reading two-dimensional bar code symbols with an elongated laser line

Abstract: A plurality of the areas of different light reflectivity of a bar code symbol, or the like, are simultaneously illuminated, using, e.g., a beam of laser light that has an elongated cross-section. The laser light beam is swept over the symbol in a direction transverse to the elongated dimension of the illuminated region so that a two-dimensional area of the symbol is illuminated over time, until the symbol is read. The light that reflects from the illuminated region of the symbol is imaged on a linear sensor array, which is then scanned or read out to produce signals representative of spatial intensity variations of the imaged light along a linear path in the field of view.

Description: Background of the Invention This invention relates to bar code readers. Typically, bar code symbols are formed from bars or elements that are typically rectangular in shape with a variety of possible widths. The specific arrangement of these elements defines a character according to a set of rules and definitions specified by the code or "symbology" used. The relative size of the bars and spaces is determined by the type of coding used, as is the actual size of the bars and spaces. The number of characters per inch represented by the bar code symbol is referred to as the density of the symbol. To encode a desired sequence of characters, groups of elements are concatenated together to form the complete bar code symbol, with each character of the message being represented by its own corresponding group of elements. In some symbologies, unique "start" and "stop" characters are used to indicate where the bar code begins and ends. A number of different bar code symbologies exist. These symbologies include, e.g., UPC/EAN, Code 39, Code 49, Code 128, Codabar, and Interleaved 2 of 5. In order to increase the amount of data that can be represented, or stored, on a given amount of surface area, several new bar code symbologies have recently been developed. One of these code standards, PDF 417 (developed by the present assignee), uses a "two-dimensional" scheme in which rows of characters are stacked vertically. That is, there are several rows of bar and space patterns, instead of only one row. Thus, bar codes include not only the traditional rectangularly-shaped bars and spaces, but any form of symbol in which different portions of the symbol have different light reflecting characteristics. Bar codes are typically scanned to transform the graphic symbol elements into electrical signals, which are then decoded into characters. A scanning system uses a light source, typically a laser, which is directed to the symbol or bar code by lenses or other optical components. The reader functions by repetitively scanning the light beam in a path or series of paths across the symbol. Scanning systems also include a sensor or photodetector which detects light reflected from the symbol. A portion of the reflected light is detected and converted into an electrical signal, and electronic circuitry or software decodes the electrical signal into a digital representation. The symbol is decoded according to the coding technique used. For example, in a Universal Product Code (UPC) bar code typical on many supermarket items, the first five digits indicate the manufacturer of the item, and the second five digits indicate the item number, which may be used by a computer to look-up the current price of the item, and to deduct that item from the supermarket's inventory. A bar code offers several advantages over other data representations. A bar code allows data to be entered into a computer more rapidly. Bar code systems are nearly error-free. Bar code formats are designed with various forms of error-checking built into the code. In recent years, there have been increasing demands for storing more information in a bar code label. In particular, there is a desire to create a "portable data file", as opposed to the "license plate" nature of conventional bar code symbols. For example, a typical UPC bar code only contains the manufacturer's code and the item number. The price look-up must be accessed in a database keyed to the item number in the bar code. A two-dimensional bar code, on the other hand, could be used to store all of the relevant information, such as price, the name of the product, manufacturer, weight, inventory data, and expiration date. This would constitute a "portable data file" because the information could be retrieved without having to access a database. While a price look-up database file is conveniently available in a retail environment, this may not be the case in other applications. Although two-dimensional bar code schemes do not have vertical redundancy, they incorporate other forms of redundancy as well as other advantages, while increasing the density of information storage. For example, Code PDF 417 allows stitching of partial scans, error detection, and error correction. EP-A-0 377 973 discloses a sensor array and illumination system for a bar code scanner using a modified Scheimpflug arrangement optical system. The preferred embodiment includes a detector array comprising a multielement array of photosensors positioned in one Scheimpflug plane and an illumination beam in the form of a sheet of light along the other Scheimpflug plane. The detector array has a plurality of elements having a long axis and a short axis, with the long axis of the elements arranged substantially perpendicular to the image of the illuminated strip of the bar code tag. The projection of the long axis of the elements of the array onto the other Scheimpflug plane defines the working range of the optical system. This arrangement requires complicated calculations and adjustments for the Scheimpflug planes. Further, the apparatus is not suitable for quick and easy reading of two-dimensional bar codes like the ones mentioned above. EP-A-0 384 955 discloses an apparatus and method for reading data in the form of indicia on a surface of a target, the indicia having a preferred reading direction, including a light source for illuminating a portion of the surface to be read with a laser beam in a scanning pattern at a predetermined scanning angle so as to scan spatially adjacent portions of the surface. A detector and a processor is provided that is operative for detecting at least a portion of the light reflected from the indicia and storing representations thereof, and further determining the angular difference between the preferred reading direction and the scanning angle. An optical component disposed adjacent the light source is provided for optically directing the laser beam along a path toward the surface and is capable of controlling and setting the predetermined scanning angle. The optical component may be automatically rotated so as to align the direction of scan with the preferred reading direction of the indicia. According to WO-A-93 17 397, an optical scanning head for scanning one- and two-dimensional bar codes includes at least one combination of light emitting diodes arranged so the LEDs emit light at different angles to create a fan of light. The combination and arrangement of the LEDs depends on whether one- or two-dimensional bar codes are to be read. A cylindrical lens optimizes the light impinging upon the bar code. An optical module includes a light shield or "dark room" and a lens/filter assembly which focuses reflected light on the detector, made up of a CCD module for detecting the light intensity in the reflected beam over a field of view across a bar code symbol. One or more combinations of linear CCD arrays in the CCD module generate an electrical signal indicative of the detected light intensity. A trigger mechanism allows selection of the number of LEDs or linear CCD arrays to scan one- or two-dimensional bar codes, where two-dimensional bar codes require more illumination for accurate scanning. Summary of the Invention The present invention features a novel method and apparatus for reading two-dimensional symbols that have regions of different light reflectivity (e. g. two-dimensional bar codes), as set forth in claims 1, 6 and 10. Preferred embodiments of the present invention may be gathered from the dependent claims. This novel reading technique can be implemented with only a small number of light sources and with a mechanically simple design, while offering a high signal-to-noise ratio. The invention provides a method and apparatus that enables the use of the same type of reader, e. g. a hand-held reader, for reading different, previously incompatible bar code symbologies. In an embodiment, certain portions or areas of a bar code symbol, or the like, are simultaneously illuminated using, e.g., a beam of laser light that has an elongated cross-sectional shape. The light that reflects from the illuminated region of the symbol is imaged on a sensor which is scanned to produce signals representative of spatial intensity variations of the reflected light along a path. The laser light beam is then moved over the symbol in a direction transverse to the elongated dimension of the illuminated region of the symbol so that another portion of the symbol can be read. The motion is either continuous or in a stepped motion so long as there is sufficient time for the reflected light to be properly imaged and processed. The beam is swept over the height of the symbol so that the entire two-dimensional area of the symbol is illuminated over time, and such motion is repeated in a back-and-forth manner until the entire symbol is read. Embodiments according to the invention require relatively low power, yet still provide sufficient illumination for reading various types of light reflecting symbols. The beam preferably diverges in an elongated cross-sectional dimension. The elongated cross-sectional dimension of the beam, in the plane of the symbol, is preferably long enough to illuminate the entirety of one dimension of a row of the symbol at one time. The beam preferably does not converge to a waist in the elongated cross-sectional dimension. The invention is particularly useful for reading various two-dimensional bar code symbols. For example, to read a two-dimensional bar code symbol consisting of a stack of parallel data rows, a bar code reader according to the invention is aligned so that the elongated dimension of the beam is generally parallel with the rows of the symbol. Preferably only one of the data rows is illuminated at one time. Alternatively, for example, matrix code symbologies, such as one formed from one or more identical shapes (e.g., hexagons), has been proposed which has a three-fold symmetry that allows the symbol to be read, according to the invention, with the elongated dimension of the beam aligned generally parallel with one of three different axes of symmetry. Other features and advantages will be apparent from the following description and from the claims. Brief Description of the Drawings Fig. 1 is a perspective view of a hand-held reader scanning a two-dimensional bar code and a one-dimensional bar code which are affixed to a surface. Fig. 2A is an enlarged view of a two-dimensional bar code symbol formed from a stack of parallel data rows illuminated by three successive light beams having elongated cross-sections. Fig. 2B is an enlarged view of a two-dimensional symbol formed from a plurality of hexagonal shapes that can be read by elongated light beams according to the invention that are aligned parallel with one of the three axes of symmetry of the symbol. Fig. 3 is a schematic perspective view of a scanning system for use with the reader shown in Fig. 1. Figs. 4A, 4B and 4C are exploded perspective and side cross-sectional views, respectively, of a light source for use with the reader shown in Fig. 1. Fig. 5A is a top plan view of an alternative scanning system for use with the reader shown in Fig. 1. Fig. 5B is a graph of the cross-sectional intensity profile of the beam shown in Fig. 5A as a function of spatial dimension along the beam parallel to the elongated dimension of the beam. Figs. 6A and 6B are cross-sectional views of different lens arrangements for use with the reader shown in Fig. 1. Fig. 7 is a schematic diagram of a detector cirouit for use in the reader shown in Fig. 1. Description of the Preferred Embodiments As used in this specification and the claims, the term "symbol" and "bar code" is intended to be broadly construed and to cover not only patterns composed of alternating bars and spaces of various widths, but also other one or two dimensional graphic patterns, as well as alphanumeric characters. Referring to Fig. 1, a hand-held reader 10 is positioned to direct a beam 12 of laser light at a surface 14 on which a two-dimensional bar code symbol 16 and a one-dimensional bar code symbol 18 are affixed. The reader is preferably implemented as a gun-shaped device with a pistol-grip handle 20. A manually actuated trigger switch 22 allows an operator to activate the reader (typically once the reader has been positioned to point at the particular symbol to be read.) A light-weight plastic housing 24 contains the optical components of the reader including a laser source and a photodetector. A light-transmissive window, at the front end of the reader, allows the beam generated by the laser to exit the reader, and allows reflected light from bar code symbols 16, 18 to enter the reader. An electrical cable 26 carries signals from reader 10 to a decode module (not shown) that decodes the signals from the reader representative of the scanned bar code symbols. An external host device (not shown), e.g., a programmable computer, serves as a data store in which data generated by the decode module is stored for subsequent processing. Each time a user wishes to read a bar code symbol, the user aims reader 10 at the symbol and pulls trigger 22 to initiate reading the symbol. Beam 12 exits the reader having a cross-sectional size with a narrow dimension 28 and an elongated diverging dimension 30. The beam preferably does not converge to a waist in elongated dimension 30. Beam 12 then sweeps over surface 14 at a rapid rate in the back-and-forth direction generally indicated by doubleheaded arrow 32 (i.e., transverse to the elongated dimension of the beam), and thereby illuminates surface 14 in a rectangular field-of-view 34 over time, as shown in phantom. As soon as the symbol has been successfully decoded and read, the scanning action is automatically terminated, indicating to the user that the reader can be directed at another symbol. As shown in Fig. 2A, a two-dimensional bar code symbol 36 is read by sweeping the laser light beam over the symbol. Over time, symbol 36 is successively scanned in the direction indicated by arrow 38, as indicated by illuminated regions 40, 42, and 44. It should be noted that some two-dimensional bar code schemes e.g., PDF417, allows stitching together of partial scans and thus do not require the illuminated regions to lie completely within a single row as shown. This is due, at least in part, to the fact that any two adjacent data rows in such schemes encode data using codewords selected from different, mutually exclusive sets. In another two-dimensional bar code scheme shown in Fig. 2B, a two dimensional bar code 46 is formed from a plurality of identical hexagonal shapes 48 which are aligned in rows parallel with three different axes of symmetry. This allows the symbol to be read by sweeping beams with elongated cross-sections over the symbol in directions generally parallel with the axes of symmetry of the symbol, as indicated by illuminating beams 50, 52, and 54. In addition to the use of an elongated static laser beam 50, 52 and 54, it is possible to use one or more static spots, e.g., as described in Heiman et al., U.S. Patent Nos. 5,250,791, 5,229,591, and 5,493,538, which are assigned to the present assignee. Alternatively, Alternatively, a combination of spots and scan lines can be used to form the aiming pattern. In addition thereto, one may use a blinking aiming beam (either spot, line or combination)) to highlight to the user that the reader is in an aiming mode. For example, if the two-dimensional bar code incorporates a bulls eye for aiming purposes, it would be useful for the aiming beam to incorporate a blinking spot to align with the center of the bulls eye, in addition to the solid line which is used for aligning with the edge of the code. See the blinking spot 300 in Fig. 2B as an example. The blinking spot 300 may be implemented in a variety of ways which are described herein for completeness. In the simplest case, the spot may be generated by a fixed laser light source which is mechanically shuttered, or activated and deactivated periodically by an electronic circuit generating a pulse signal. A more complete implementation would provide a "spot" beam that would alternate between a static spot and a short line. Such an implementation would provide a pulse signal of different frequencies both to the laser diode and a scanning mirror so as to achieve the alternation between a spot and a narrow line. The illuminated beams 50, 52 and 54 may be produced by a single scanning arrangement which is angularly repositioned after each scan, or by separate independent scanning arrangements that are spatially positioned to produce the three laser light beam lines shown in Figure 2B. Figure 3 illustrates a single independent scanning arrangement in detail, but it should be recognized that two, three or more such arrangements may be implemented in a single reader to be able to produce the multiple line pattern such as shown in Figure 2B. Note that orthogonal beams 52 and 56 may be utilized by a user to "frame" the symbol, and so position and orient the reader so as to align the static laser beam parallel to an edge or axis of symmetry of the symbol. This is especially important in the case of a hand-held reader, such as shown in Figure 1. Before describing the scanning arrangement of Figure 3, the operational modes can be discussed in greater detail. The features of the present invention may be utilized to operate the reader in different modes. There are three possible modes that we might consider for a hand-held reader according to the present invention: (a) the normal triggered mode; (b) the triggered spot and scan mode; and (c) the object sensor mode. In the normal triggered mode, the laser beam is initially off. A trigger switch is used in the triggered mode to initiate both activating the static laser beam and the rapid and repetitive scanning of the target symbol. In many applications, it is necessary to distinguish between whether many scans have been performed for one object, or whether one scan has been performed for many objects. The capability of sensing each object to be scanned in its turn is critical for successful decoding. As is known in prior art scanners (such as described in U.S. Pat. No. 4,387,297) a manual trigger may be used for actuating the light source and/or the scanning means to repetitively sweep the barcode symbol a number of times each time the trigger is actuated. The trigger is preferably a manually-depressible switch mounted on the housing in the vicinity of the interconnection of the barrel and handle portions of the housing. The trigger switch is located on the handle portion such that the forefinger of the user's hand can be used to manually depress the switch. Each time the switch is depressed, the laser beam is activated, and the scanning means sweeps the symbol many times, until a complete decode of the entire symbol or a time out is reached. Either a single-position trigger or a multi-position trigger may be used. In the normal triggered mode, when the decode circuitry successfully decodes the symbol, the decode circuitry generates a successful decode signal and may actuate the indicator means located in the scanner. The indicator means comprises an auditory-type beeper and/or a light-emitting diode. When the beeper sounds and/or when the diode lights up, then the user knows that the scanning for that particular symbol has been terminated. In the triggered spot and scan mode, after the trigger is pulled, the beam comes on a fixed or static mode. In such an operational mode, one or more very bright lines, spots or patterns are formed by the optics from the laser beam (i.e., the "spot" mode). The bright beam is used by the user holding the laser reader to manually aim and direct the beam to the location where the user actually sees the barcode is located. Once the reader has been properly positioned, the user may pull the trigger to begin the scanning process (i.e., the "scan" mode). As an alternative to triggered operation, in the automatic spot and scan mode, when an indicia pattern indicative of a barcode symbol has been detected by software, the beam will automatically begin to sweep the entire symbol so that it can be decoded. In the object sensor mode, the reader is triggerless and the constantly on fixed or static laser beam is used to allow the user to position the reader. When an indicia pattern indicative of a barcode symbol has been detected, the beam will begin sweeping the symbol until the beam reads the entire symbol. Any or all of the above operational modes may be utilized in a single reader utilizing the bar code symbol detection techniques known in the art. Such operational modes may be chosen manually by the user (such as by setting a switch) or may be automatically determined and dedicated in a particular type of reader. The particular type of scanning parameter may also conceivably be automatically determined by an analysis of the detected bar code and a computer algorithm may determine the most appropriate subsequent scanning mode to be utilized in order to most effectively scan and detect the entire bar code symbol. Such alternative embodiments are all within the range and scope of the present invention. In the preferred method of reading according to the present invention, a two-dimensional bar code symbol is presented having a plurality of ordered rows of bar-coded information, each row extending generally along a longitudinal direction, with the rows being successively arranged along a transverse direction generally perpendicular to the longitudinal direction. The symbol is illuminated with a first longitudinal beam of light extending in a first direction, and a second longitudinal beam of light extending in a second direction orthogonal to the first direction, by the reader. The user then manually positions the reading device so that at least one of the beams is aligned parallel to the row of the symbol. Light reflected over a field of view extending along the longitudinal direction is detected, and a first electrical signal indicative of the detected light over the longitudinal direction is generated. The first electrical signal is processed by software to determine is the reflected light in the longitudinal direction represents one of the rows. The detection and processing steps are repeated until the longitudinal direction corresponds to one of the rows, thereby signifying that the reading device is properly aligned with the bar code. Upon proper alignment, the sweeping motion is initiated. A relative movement between the field of view and another of the rows along the transverse direction is performed, either automatically by a mirror, or by mechanical motion. Light is reflected off said another of the rows, and detected, generating a second electrical signal indicative of the detected light over such rows, and decoding the second electrical signal to produce data represented by such row. The detection and decoding steps are repeated until data in each of the rows has been decoded so that the entire two-dimensional bar code symbol has been decoded. Referring to Fig. 3, a laser line generator 60 emits a laser light beam 62 that diverges in one dimension (and over a certain distance converges in the other) toward optical component 64 (e.g., an oscillatable mirror or lens), which in turn directs the fixed, static beam 12 toward the bar code symbols to be read. As beam 12 illuminates the surface 14, light reflecting from a portion of the bar code symbols 16 is collected and directed to a lens 66. Lens 66 preferably focuses the collected beam of light onto a one-dimensional charge-coupled device (CCD) sensor 68. The focused light 70 has a cross-sectional size with a long dimension and a narrow dimension which correspond to the long and narrow dimensions of beam 12, respectively. This correspondence allows the CCD to scan the field of view and generate signals representative of the spatial intensity variations in the light reflected from the illuminated portion of the bar code symbol in the field of view, such as a row or linear group of modules. An optical lens and an optical filter can be mounted inside the reader and positioned to intercept the reflected incoming light before the light reaches the detector. If a filter is used, it is desirable that the filter only selectively transmit light having a wavelength range corresponding to the wavelength range of the scanned laser beam. An electrically-operated drive 72 preferably moves component 64 in alternating circumferential directions to produce back-and-forth sweeping of beam 12 across surface 14 over a field-of-view 34. In an alternative embodiment, the drive causes the component to rotate completely around an axis of rotation 74. Alternatively, the scanning optics may comprise an omni-directional scan pattern generator that generates intersecting laser line patterns, as described in Krichever et al., U.S. Patent No. 5,059,779 (assigned to the present assignee). According to this scheme, a beam that has an elongated cross-section can be swept over the symbol in different reading directions until the elongated dimension of the beam is properly aligned with the symbol and the symbol can be read. For example, the scan pattern generator can be used to successively produce illuminating beams 50, 52 and 54 shown in Fig. 2B. Figs. 4A-4C illustrate the construction of the laser line generator 60. As shown in Fig. 4A, a laser diode 84 generates a light beam which has a generally elliptical cross section, and which in the near field (close to the laser diode) has a cross section 100 with a particular size, shape and orientation. A cylindrical lens 88 is positioned adjacent the laser diode 84 so that the long axis of the lens 88 corresponds to the long axis of the elliptical cross section 100. The lens 88 functions to broaden or diverge the beam 100 in the direction of the elongated axis of the ellipse producing a very narrow, elongated beam 101. Fig. 4B shows the packaging of the assembly in the laser line generator 60. Referring to Fig. 4C, which depicts a cross-section of the assembly 60, laser beam 62 exits laser line generator through an aperture 80 and a window 82. A laser diode 84 is seated in laser holder 86, and a cylindrical or toric lens 88 is seated in lens holder 90. Between these two holders is a threaded interface 92. Lens holder 90 is urged away from laser holder 86 by spring 94. During manufacture, the laser line generator is focused by sliding lens holder 90 with respect to laser holder 86. As soon as the desired focus is achieved, the two holders are permanently fixed relative to one another by a variety of possible techniques, including using adhesives such as glue or epoxy, fastening by staking, spot welding, and ultrasonic welding. Referring to Fig. 5, an alternative reader 110 employs two laser line generators 112, 114. A scanning component 116 (e.g., a rotating mirror) directs the beams generated by generators 112, 114 toward the target to be read through window 118. The light reflected from a target is collected by lens 120 which in turn focuses the collected light onto a detector 121, e.g., a 1024 pixel linear CCD array. Two laser line generators are used in this embodiment due to the fact that the irradiance distribution along a laser beam from a single source is not uniform, but instead decreases rapidly from the center of the beam in a manner generally following a Gaussian distribution. Summing together the outputs from two laser line generators provides a more uniform intensity distribution along the resulting laser line. Referring to Figs. 5A and 5B, a laser beam line 122 resulting from the optical summation of the beam outputs 124, 126 from laser line generators 112, 114, respectively. As shown in Fig. 5B, the uniformity of the intensity profile across the elongated dimension of beam 122 is significantly improved relative to the intensity profile of only one of the beams alone, as shown in phantom. The combination of the outputs from more than one source enables a longer laser line to be produced which has a resulting intensity that is sufficient to adequately illuminate a symbol 126 for reading, thereby providing reader 110 with an elongated field of view. In an exemplary embodiment of reader 110, the cross sectional line length along long the dimension of the resulting beam varies from about five to ten inches at respective distances from window 82 of about four to twelve inches. Figs. 6A and 6B are cross-sectional views of preferred embodiments of the detector lens 120 which is selected to provide a flat image field and semi-wide angle field of view. A Tessar lens 128, shown in Fig. 6A, formed from a positive lens 130, a negative lens 132, a pinhole aperture 134 and a cemented doublet 136, can be used. Alternatively, lens 138, which is a simplified version of lens 128, can be used. Lens 138 is formed from a positive lens 140, a pinhole aperture 142, and a cemented doublet 144, and has a similar performance as lens 128. Turning next to Fig. 7, there is shown a schematic diagram of an embodiment of a preferred detector-circuit 217. The signal from the amplifier/digitizer 216 is applied to the input of an inventor 231, the output of which is applied to a diode 232. An RC circuit is formed by first resistor R1 and capacitor (C) 234 is connected between the first input 235 and ground potential, and a second resistor R2 is connected between first input 235 and ground potential. The resistance value of resistor R1 is preferably much less than that of resistor R2. The second input 238 of the comparator is connected to the mode of a voltage divider formed by resistors R4 and R5 which are connected in series between potential V and ground. The output 241 of the comparator 237 is connected to the "laser enable" signal line, as well as a feedback through a resistor R3. The feedback from the output of the comparator 237 provides a hysteresis effect to the comparison operation since the other terminal of the resistor R3 is connected to the second output 238 of the comparator 237. The operation of the detector circuit 217 can be described as follows: when the digitizer outputs a bar, the capacitor charges up with a time constant of approximately R1C since R2 is much greater than R1. When the digitizer outputs a space, the capacitor discharges through R2 since the diode 232 prevents the discharging through R1. The time constant R2C is much greater than the time constant R1C so that more space time is required to cancel the effect of a bar. After several bars and spaces of typical density, a voltage is developed on the capacitor 234 which exceeds the threshold which has been set with the user of the comparator 237. At this time, a "trigger" or laser enable signal is output from the comparator 237 to indicate the presence of a bar code. The open collector output of the comparator 237 is driven low at this point which lowers the threshold of the comparator so that minor voltage changes on the capacitor 234 due to the following bars and spaces, and quiet zone, will not disable the trigger signal. The circuit as described would also trigger if a long black bar were to be scanned. However, in the preferred embodiment, the digitizer includes a circuit which discriminates against reading a long black bar, i.e., the digitizer functions as a high pass filter. one such digitizer circuit would utilize a time-out so that if a long black bar was scanned, only a short pulse would be generated. When such a short pulse signal is applied to the detector circuit 216, the threshold will not be exceeded and a "trigger" signal will not be output. The trigger signal will only be released after a relatively long time during which there are no bars digitized. When the scanner is moved away from a symbol, the capacitor will discharge through R2C and the trigger signal will be released which will indicate to the decoding logic or the detector circuit that the same symbol is no longer being scanned. The bar code sensing circuit depicted in Fig. 7 is sensitive to a specific pattern of images. The circuitry is responsive to a pattern of high and low reflected light signals to charge and discharge the capacitor 234, with the net charge being utilized to generate a trigger signal after some threshold has been reached. The circuit described in Fig. 7 is useful to discriminate a barcode in the scan field from text or other graphics. To do this it exploits the single dimension aspect of a bar code to differentiate it from text. It relies on motion of the scanline, which ordinarily would come from motion of the scanner in a user's hand, to compare different slices of the pattern in the scanfield. If successive slices were similar, within limits determined by the implementation, it was highly probable that a barcode was being scanned. If successive slices were dissimilar, it was likely that a barcode was not being scanned. This algorithm is modified to allow two dimensional bar code to be discriminated from graphics. It relies on the one dimensional nature within regions in the Y dimension.

Claims: A method for reading a two-dimensional symbol (16, 36, 46) representing information having areas of different light reflectivity formed from substantially identical shapes that are aligned in rows parallel with three different axes that intersect each other at a point to form acute angles of 60° therebetween, comprising the steps of: providing a beam (50, 52, 54) of laser radiation the beam having an elongated cross-sectional shape with an elongated dimension (30) and a narrow dimension (28); simultaneously illuminating a region including a plurality of the areas of different light reflectivity of the symbol with said beam; aligning said beam so that the elongated dimension of said illuminated region is substantially parallel to one of said three different axes; imaging light reflecting from the illuminated region to produce a signal representative of spatial intensity variations of the imaged light within said illuminated region; determing that said signal represents one of the rows of said symbol; and sweeping said beam over the symbol in a direction transverse to the elongated dimension of the illuminated region so that substantially the entire two-dimensional area of the symbol is illuminated over time, until the information represented by the symbol is read. The method of claim 1 wherein the elongated cross-sectional shape of said beam is imparted to said beam by receiving a source laser beam from a semiconductor laser diode and optically broadening the larger cross-sectional axis of said source laser beam in the elongated axial dimension. The method of claim 2 wherein said broadening comprises the step of diverging said beam in the elongated dimension by a lens. The method of claim 2 wherein in said elongated dimension said broadened beam does not converge to a waist . The method of claim 1 wherein said step of illuminating comprises illuminating a region of the symbol that includes the entirety of one dimension of the symbol at one time. A method of reading with a hand-held reading device (10) a two-dimensional code symbol (16, 36, 46) having a plurality of ordered rows of coded information, each row extending generally along a longitudinal direction, said rows being successively arranged along a transverse direction to the longitudinal direction, comprising the steps of: (a) illuminating the symbol with a first elongated beam (52) of light extending in a first direction, and a second elongated beam (56) of light extending in a second direction orthogonal to the first direction; (b) manually positioning the device so that one of said beams is aligned parallel to a row of said symbol; (c) detecting light reflected over a field of view extending along the first and second directions, respectively, and generating a first electrical signal indicative of the detected light; (d) processing the first electrical signal to determine if the reflected light represents one of the rows of said symbol; (e) repeating said steps (b) to (d) until the reflected light corresponds to one of the rows; and (f) once the reflected light corresponds to one of the rows, sweeping at least one of the beams over the symbol so as to read the symbol. A method as defined in claim 6, wherein said sweeping step comprises sweeping at least one of said beams over the symbol in a direction transverse to the elongated dimension of said swept beam so that the entire two-dimensional area of the symbol is illuminated over time, until all of the information of the symbol is detected and read. The method of claim 6 or 7 wherein an elongated cross-sectional shape of each of said beams is imparted to each of said beams by receiving a source laser beam from a semiconductor laser diode and optically broadening the larger cross-sectional axis of said source laser beam in the elongated axial dimension. The method of any of claims 6 to 8 wherein the rows are parallel to three different axes that intersect each other at a point to form acute angles of 60 degrees therebetween, and the direction generally transverse to the longitudinal direction is 60 degrees from the longitudinal direction. An optical reader (10) for reading a two-dimensional symbol (16, 36, 46) having areas of different light reflectivity arranged in rows, comprising: means (60) for providing a beam (12, 62) of laser radiation; means (88) for simultaneously illuminating with said beam a plurality of the areas of different light reflectivity at the same time in an elongated, illuminated region of the symbol; means (70) for imaging the light reflecting from the illuminated region to produce a signal representative of spatial intensity variations of the imaged light along said region; characterized by means for determing that said signal represents one of the rows of the symbol; and means (64, 72) responsive to the determing means, for sweeping said beam back-and-forth over the symbol in a direction transverse to the elongated dimension of the illuminated region so that a two-dimensional area of the symbol is illuminated over time, until the symbol is read. An optical reader as defined in claim 10, wherein said means (70) for imaging comprises an optical detector constructed and arranged to receive light reflected from the illuminated region of the symbol to produce a signal representative of spatial intensity variations of the reflected light, and said means (64, 72) for sweeping comprises an optical scanning component for sweeping said beam back-and-forth over the symbol. The reader of claim 11 further comprising optical elements (88) that cause the cross-sectional size of said beam to diverge in the elongated dimension. The reader of claim 12 wherein said optical elements (88) comprise a cylindrical lens. The reader of claim 12 wherein said optical elements comprise a cylindrically-shaped mirror. The reader of claim 11 wherein said scanning component comprises a mechanical drive (72) for sweeping said beam over said symbol. The reader of claim 15 wherein a mechanical drive (72) rotates a lens or mirror (64) for sweeping said beam over said symbol. The reader of any of claims 10 to 16 wherein said beam comprises visible laser light. The reader of claim 11 wherein said detector (70) comprises an array of individually accessible photodetectors. The reader of claim 11 wherein said detector (70) comprises a CCD array. The reader of any of claims 10 to 19 further comprising a hand-held housing constructed and arranged for containing all of said means.

Inventors: DVORKIS PAUL, LI YAJUN, SPITZ GLENN, SWARTZ JEROME, DVORKIS, PAUL, LI, YAJUN, SPITZ, GLENN, SWARTZ, JEROME
Applicants: SYMBOL TECHNOLOGIES INC, SYMBOL TECHNOLOGIES INC.
Application year: 1994
Main classification: G06K7
IPC codes: G06K7